1. Field of the Invention
The present invention relates to DC component cutoff filter circuits or high-pass filter circuits and, more particularly, to a filter circuit utilizing a capacitance amplifier circuit (C multiplier).
2. Description of the Related Arts
As a capacitance connected to a DC cutoff filter circuit or the like as shown in FIG. 2, a capacitance amplifier circuit can be used, wherein a composite capacitance is formed using an operational amplifier as shown in FIG. 3A. The capacitance amplifier circuit (FIG. 3A) can be considered as a circuit which equivalently amplifies a capacitance by a factor of R1/R3 when resistance R3 is sufficiently smaller than resistance R1. This circuit is widely used because it can be provided utilizing a capacitor having a capacitance smaller than that which is actually needed and it can be made compact as a whole even when it must have a large capacitance. The filter circuit shown in FIG. 2 is constituted by a CR circuit having a composite capacitance and series resistance R4 (the equivalent circuit is shown in FIG. 3B), and its output is current-amplified by a voltage follower of an operational amplifier A2.
However, the capacitance amplifier circuit shown in FIG. 3A has a problem in that even if the operational amplifier A1 used is of a high performance type having a low drift, a drift in the capacitance output voltage is caused by a temperature drift in its offset voltage and offset current, a temperature drift of the capacitor used therein itself and a temperature drift of the resistor used therein itself. This results in a problem that the drift adversely affects the characteristics of the filter, especially the output characteristics when the input signal is very small, thereby reducing the reliability of the device as a whole.
One solution to this problem is to use a resistor having a different drift to compensate for the temperature drift of the capacitor. However, a problem still remains in this case that even though the temperature drift in the capacitor can be canceled, drifts in DC components due to the operational amplifier can not be completely canceled and, as a result, the characteristics of the circuit are not satisfactory especially as those of a filter circuit wherein highly accurate signal processing must be performed.